1. Field of the Invention
The present invention relates to a living body stimulating apparatus. More particularly, the present invention concerns a stimulating apparatus of the kind comprising a conductor element with a built-in electrode which can be attached to a living body. Stimulation is achieved by an electric current flowing through the electrode into the living body.
2. Description of the Related Art
A conventional living body stimulating apparatus is used for the treatment of a nerve function in a diseased part, by allowing a low frequency pulse current to flow in the diseased part, such low frequency pulse current being output from a transmitter to an electrode. In Japanese Patent Un-examined Publication No. 1-146562, for example, is disclosed a living body stimulating apparatus which enables the control of the speed and intensity of stimulation, by switching stimulation signals output from an output circuit to a living body (or human body). The stimulation signals are either DC intermittent pulses for periodically outputting positive pulses, AC intermittent pulses for periodically outputting rectangular wave pulse groups consisting of positive pulses and negative pulses, alternate intermittent pulses for periodically and alternately outputting positive pulses and negative pulses, the period or amplitude of which can be varied.
However, it is generally difficult to make pass a DC current through a human body, since it has a resistance of about 100 KΩ, depending upon voltage, whilst a high-frequency AC current flows easily through a human body. For example, a human body exhibits a resistance of about 1 KΩ under an AC voltage of 1 KHz, the resistance being reduced by half if the frequency doubles. In other word, a human body has a capacitive impedance and thus the in-vivo resistance tends to decrease as frequency increases. On the other hand, as the stimulation of the human body is concerned, low frequencies close to direct current, and rectangular waves comprising a high DC component are more stimulative. Thus, for the same frequency, sinusoidal waves will induce softer stimuli than rectangular waves.
FIG. 7 illustrates an example of a conventional living body stimulating apparatus for applying stimulation signals having the form of sine waves to a human body. As shown, numeral 101 designates a CPU (central processing unit) as controlling means. The CPU outputs digital data signals which are converted into analogue data signals by a D/A converter circuit 102. Then, the analogue data signals are amplified by an amplifier 103, to provide sine-wave stimulation signals across conductive elements or output electrodes 105, via a transformer 104. The amplitude of the sine wave can be increased or decreased arbitrarily by operating a variable gain element 106 at the input of the amplifier 103.
Although a sinusoidal wave induces a soft stimulation of the human body and is agreeable thereto, it comprises only one frequency component, which leads to a very limited therapeutic effect. Furthermore, the output circuit of such a conventional apparatus requires analogue circuits such as the D/A converter 102 and the amplifier 103 which are necessary for obtaining a substantially sinusoidal waveform. The number of components is relatively high and the circuitry is complex, thus leading to low power efficiency and high manufacturing costs. In other words, conventional circuits outputting sinusoidal waves would require dozens of components such as transistors, resistors and capacitors.
In an effort to solve these problems, Japanese Patent Un-examined Publication No. 2001-259048 previously filed by the present applicant proposed a living body stimulation apparatus. The conventional apparatus is provided with a stimulus generator means for applying pulse-width modulation to rectangular wave pulses that are output at predetermined recurrence frequency, and then recurringly outputting rectangular wave pulse groups containing a plurality of higher frequency components than the rectangular wave pulse to the electrodes as stimulation signals. In the conventional apparatus, PWM is adopted so that a time width of each on-pulse gradually increases during the first half of the rectangular wave pulse group and then gradually decreases during the second half thereof as it comes closer to the falling edge of the rectangular wave pulse group. As a result, the waveform of the rectangular wave pulse group applied to a human body is distorted to form an approximately sinusoidal wave, which can give soft feeling of stimulation to a human body.
In the above second Patent Publication, the time width of each on-pulse constituting the rectangular wave pulse group is gradually increased and then gradually decreased by means of PWM, and then, thus modulated rectangular wave pulse group which has alternately a positive voltage or a negative voltage, is applied to a human body to be distorted by a capacitive element of the human body, so that stimulation signals are deformed into waveforms approximate to sinusoidal waves of low frequency, thus giving soft feeling of stimulation. Since the time width of each on-pulse, however, is varied for example in a range of from 10 μsec to 60 μsec during the generation of the rectangular pulse groups, the time width of the on-pulse is likely to becomes too large partially, so that a charged quantity in equivalent electrostatic capacity of a human body rises rapidly, resulting in a drawback that cenesthesia of soft stimulation is hard to obtain.